mongo调研

https://docs.mongodb.com/manual/core/replica-set-sync/
https://docs.mongodb.com/manual/tutorial/add-replica-set-arbiter/
https://www.cnblogs.com/66li/p/12058750.html
https://www.docs4dev.com/docs/zh/mongodb/v3.6/reference/reference-method-db.getSiblingDB.html
https://mongodb.github.io/mongo-java-driver/3.4/driver/tutorials/connect-to-mongodb/
https://soulchild.cn/1939.html
https://www.jianshu.com/p/31519bc69377
https://docs.mongodb.com/manual/core/replica-set-architecture-geographically-distributed/
https://www.jianshu.com/p/ac31be1fef1f/